(57) A compound of the formula;
 wherein R1 is an amino group which may be protected, R3 is a hydrogen atom or an optionally substituted hydrocarbon residue; Z is S, S →
O, O or CH2, R4 is a hydrogen atom, methoxy group or formamido group, R13 is a hydrogen atom, methyl group, hydroxyl group or halogen atom and A⊕ is an optionally
substituted imidazolium-1-yl group forming a condensed ring at the 2,3- or 3,4-position
or a pharmaceutically acceptable salt or ester thereof is novel and has excellent
antibacterial activity.
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